The Kondo insulator SmB6 exhibits a mysterious resistivity saturation with a residual conductivity at low temperature. All efforts over the years to explain this unusual behavior have failed. The recent theory suggests this highly correlated material is an interaction induced topological insulator with the residual conductivity originated from robust surface states. Transport measurements have established the presence of an intrinsic metallic surface and fully insulating bulk state at the low temperature. Following a brief introduction of physics of Kondo lattice and properties of Kondo insulators, I will describe the experimental results providing SmB6 a necessary condition of topological Kondo insulator. Other recent experiments in which the surface states were detected will be surveyed.